This invention relates to a roller follower with a roller adapted for rolling contact with a cam on a camshaft of an engine, and more specifically to a roller follower which exhibits a long life irrespective of the lubricating conditions with respect to the cam surface of the camshaft, and the hardness and surface roughness of the cam surface.
In improving the durability of the engine valve system and keeping them maintenance-free, lubrication and wear of a cam and a roller follower present problems.
To prevent wear of the valve system, a hydraulic lash adjuster is usually used in an OHV type engine. Also, in order to reduce wear and friction loss on the cam surface and the roller follower surface, there is a growing tendency to adopt a rollable roller follower.
Among the engine parts, the lubrication requirement is especially strict for such cam members. Their contact surface is called a boundary lubrication region. With a roller follower used under such conditions, its roller basically makes rolling contact with respect to the cam. However, since the rotating speed of its roller changes due to the shape of the cam or the load acting on the roller fluctuates sharply, it cannot make a pure rolling motion but makes rolling contact accompanying sliding motion. This may cause peeling on the outer peripheral rolling surface of the roller, though depending upon the lubricating conditions (amount of oil, oil temperature, and foreign matter) and the surface roughness of the mating cam, thus shortening its life.
It is well-known that the rolling life of the roller is greatly influenced by the surface roughness of its outer peripheral rolling surface. Heretofore, it has been thought that the more smoothly the outer peripheral rolling surface is finished, the longer the life. But, the finished surface of the cam surface usually has an Rmax (parameter of surface roughness) of 2-4 microns when finished by grinding. This is not satisfactory as a rolling contact surface. At the contact portion between the cam surface and the roller, which is a rather strict boundary lubrication region, metal-to-metal contact partially occurs, making the parts liable to surface breakage. Thus, it is desired to increase the durability of the roller by preventing it from getting damaged.